Thermally processable imaging elements, including films and papers, for producing images by thermal processing are well known. These elements include photothermographic elements in which an image is formed by imagewise exposure to light followed by development by uniformly heating the element. Such elements typically include photosensitive silver halide, prepared in situ and/or ex situ, as a photosensitive component, in combination with an oxidation-reduction image forming combination, such as silver behenate with a phenolic reducing agent. Such elements are described in, for example, Research Disclosure, June, 1978, Item No. 17029, U.S. Pat. Nos. 3,457,075; and 3,933,508.
Photothermographic elements are typically processed by a method which comprises imagewise exposure of the element to actinic radiation to form a latent image therein followed by heating of the imagewise-exposed element to convert the latent image to a visible image. The simplicity of this method is highly advantageous. However, photothermographic elements are subject to several limitations which restrict their usefulness, including concerns relating to keeping stability and dye stain.
Silver halides have an intrinsic spectral sensitivity to blue light. Photographic emulsions can be spectrally sensitized to other wavelengths through the process of spectral sensitization with spectral sensitizing dyes. The spectral sensitivity corresponds to the absorption spectrum of the absorbed spectral sensitizing dye. The absorption spectrum is determined by the chemical substituents of the dye.
Photothermographic elements have been described heretofore in numerous patents, many of which teach the use of spectral sensitizing dyes. Examples of such spectral sensitizing dyes include the cyanine dyes of U.S. Pat. Nos. 3,457,075, 3,719,495, 3,761,279, 3,877,943, 4,156,611, 44,461,828 and 5,393,654. There is, however, still a critical need in the art for improved photothermographic elements utilizing spectral sensitizing dyes with enhanced performance characteristics.
There are a number of serious problems associated with the current use of spectral sensitizing dyes in photothermographic elements. Thus, for example, many sensitizing dyes spectrally sensitize over broad bandwidths but with a relatively low degree of spectral sensitivity. The low spectral sensitivity of these dyes necessitates the use of relatively large silver halide grains to achieve photographically useful speeds. However, large grains tend to cause excessive visual Dmin due to light scattering. This problem can be avoided by the use of finer grains, but then a more efficient spectral sensitizer is needed to prevent a corresponding speed loss. A further problem is that spectral sensitizing dyes can cause undesirable dye stain in the processed element. Both excellent keeping stability and low dye stain are desired in addition to efficient spectral sensitization.
It is toward the objective of providing improved photothermographic elements, sensitized with spectral sensitizing dyes that combine highly efficient sensitization with excellent keeping stability and low dye stain, that the present invention is directed.